The present invention relates to polymer electrolyte fuel cells (PEFCs).
FIG. 1 is a diagram illustrating a typical PEFC. In FIG. 1, a membrane electrode assembly (MEA) 150 is sandwiched between a fuel delivery system 120 and an oxidizer delivery system 180. The MEA 150 includes a polymer electrolyte membrane (PEM) 155, an anode catalyst layer 154 on an anode surface of the PEM 155, an anode diffusion layer 153 covering the anode catalyst layer 154, a cathode catalyst layer 156 on a cathode surface of the PEM 155, and a cathode diffusion layer 157 covering the cathode catalyst layer 156.
The PEM is a proton-permeable, electrically non-conductive membrane that allows protons to travel through the PEM from the anode to the cathode while preventing electrons from passing through the PEM. An example of a PEM typically used in fuel cells is a perfluorosulfonic acid membrane such as sulfonated tetrafluorethylene copolymer membranes available as Nation® plastic membrane from E.I. Dupont de Nemours and Company of Wilmington, Del. The anode catalyst layer 154 includes a catalyst such as platinum for increasing the anode reaction rate. The anode diffusion layer 153 is typically a porous electrical conductor such as carbon paper or cloth that conducts electrons generated by the anode reaction from the anode catalyst layer 154 to an external load while allowing transport of anode reaction reactants and products between the anode catalyst layer 154 and fuel delivery system 120. The cathode catalyst layer 156 includes a catalyst such as platinum for increasing the cathode reaction rate. The cathode diffusion layer 157 is typically a porous electrical conductor such as carbon paper or cloth that conducts electrons from the external load to the cathode catalyst layer 156 while allowing transport of cathode reaction reactants and products between the cathode catalyst layer 156 and the oxidizer delivery system.
Fuel delivery system 120 delivers fuel 123 to the anode catalyst layer 154 and removes reaction products 127, if any, from the anode. Fuel delivery system 120 may include a flow distributor that distributes the fuel evenly over the anode side of the MEA, a reformer when methanol is the fuel, a humidifier to control water content at the anode, and valves and pumps to control the flow of materials into and out of the anode. Typically, the reformer, humidifier, and pumps are housed external to the fuel cell but contribute to the overall portability of the fuel cell, in a hydrogen fuel cell, where the fuel is hydrogen, no reaction products are produced at the anode side. When the fuel is methanol, the methanol can either be converted to hydrogen using a reformer or can be applied directly to the anode. When a reformer is used, water must be supplied to the methanol and carbon dioxide removed from the reaction products. The reformer and its associated water management system adds bulk to the overall fuel cell and reduces the portability of the fuel cell. When methanol is fed directly to the anode, the fuel cell is called a direct methanol fuel cell (DMFC) and water must be supplied with the methanol to the anode catalyst layer at the anode. If sufficient water is not provided at the anode, the methanol may be incompletely oxidized to form reaction products such as formaldehyde or formic acid. The incomplete oxidation of the fuel reduces the energy generated by the fuel cell and decreases the efficiency of the fuel cell.
Oxidizer delivery system 180 delivers oxidizer 183 to the cathode catalyst layer 156 and removes reaction products 187 from the cathode. Oxidizer 183 is generally oxygen and may be conveniently provided as air although pure oxygen or enriched air may be used as the oxidizer 183. Protons from the anode recombine with the oxidizer at the cathode to produce water as a cathode reaction product 187. The water produced at the cathode may be supplied to the fuel delivery system in a DMFC. If water removal from the cathode is inefficient, cathode catalyst flooding may occur where excess liquid water coats the catalyst particles and reduces the ionization of the oxidizer at the cathode. If too much water is removed from the cathode, the PEM may dry out and reduce the conductivity of protons through the PEM. Oxidizer delivery system may include a flow distributor that distributes the oxidizer over a cathode surface of the MEA, a humidifier, a water reservoir, and pumps, blowers, and valves to control the material flows to and from the cathode. Typically, the humidifier, reservoir, pumps, and valves are housed external to the fuel cell but contribute to the overall hulk of the fuel cell.
The use of external pumps, humidifiers, and reservoirs detract from the portability advantage of a DMFC. Therefore, there remains a need for DMFC systems having reduced numbers of external components for increased portability.